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of my work comes from wholesalers, so I get boxes of them without any reference to what the customer's problem was.
In general, they have any combination of the following due to solder joints:
A few % lean due to bad solder joints in the power supply
Full lean intermittance on Map sensor
One or both O² circuits not tracking properly
Dead or intermittent (chattering) fuel pump circuit
“A rough guess is that about 10% have only a bad fuel pump circuit. Out of these I'd say that on more than 80% you
can induce one or more of the other failures by just flexing the circuit board.”
“You shouldn't pay more than $100 for a used 6CU unless it's been rebuilt by someone who knows what they're doing.
The best replacement would be a 16CU. They are a much safer buy "used" than the 6CU's.” Not only do the 16CU’s
appear to be far more reliable, they also have significantly better processing and programming built in.
HOT ECU’S: No, not stolen. Walter Petermann reports that some 16CU’s have microprocessors inside that get very
hot when running. Apparently several different type microprocessors were used, and one type in particular pulls half an
amp and therefore gets quite toasty. The good news: apparently it doesn’t hurt anything; while the 16CU has the
occasional microprocessor malfunction just like any comparable electronic equipment, there’s no significant history of
heat-related failure. Just the same, Petermann likes to take one of two actions when he finds one of these hot ECU’s.
His preferred fix is to replace the microprocessor with one of the type that doesn’t get so hot. “Unfortunately the thing
has 64 legs and they are soldered on both sides (and through the hole) of the board. Replacing them takes almost as
long as rebuilding a 6CU. It's something you never seem to get comfortable doing. There's traces running between the
pads on both sides of the board. If someone wants to try it, the replacement chip is an NEC D78C10AGQ.”
His other fix is to install a 12V cooling fan (commonly available for cooling computer CPU’s) inside the ECU. The fan
is installed entirely within, there are no openings made in the case; it merely circulates the air around inside the box,
thereby moving the heat from the chip to the aluminum case more efficiently. “Yes, it's a PC fan. It comes with a
finned aluminum heat sink and claws to attach to a chip. Unfortunately the uP in the ECU is not the correct size, so the
heat sink is not used and must be removed. You can glue the fan in place on the side of the ecu about 1/4" above the
microprocessor. There's a convenient small ledge running along the center line of the ecu case that it can also be glued
on for extra strength. Power can be taken from the filtered 12V source of ECU pin #22. This is on the yellow 'tubular'
cap C304 on the end that's close to the center-line of the ECU. Ground goes to pin #35.”
THROTTLE POTENTIOMETER: If your XJ-S has ragged throttle response as though it has a bad accelerator pump,
here’s some news for you: it doesn’t have an accelerator pump. However, the throttle potentiometer, located
underneath the throttle pulley on top of the engine, can cause similar symptoms when bad. When operating properly,
the resistance across this pot varies smoothly as the pulley is rotated. If it is breaking up, it confuses the EFI computer
on accels. The EFI computer, using inputs from the oxygen sensors, can usually keep the engine running reasonably
well at constant throttle, but it stumbles during throttle changes.
The original 1980-88 pot, 73200 or EAC2670, is both unreliable and expensive. Sounds like a Lucas part, but it’s
actually made by Bourns. Clearly, simply stamping “Lucas” on the top has an effect on reliability!
THROTTLE POTENTIOMETER ADJUSTMENT: When you replace the throttle pot, the Jaguar manual says you
must adjust it using their special electronic tester. Below is the alternate method.
The throttle pulley assembly must be unbolted from its tower to adjust the pot, meaning the linkages to the butterflies
are disconnected; but the idle stop is part of the throttle pulley assembly and therefore is unaffected. You can even start
the engine in this condition, as long as you don’t go above idle.